This invention relates generally to nuclear reactors and more particularly, to core plate and reactor internal pump differential pressure lines for a boiling water reactor.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core plate and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. The core center axis is substantially coaxial with the center axis of the shroud, and the shroud is open at both ends so that water can flow up through the lower end of the shroud and out through the upper end of the shroud. The shroud, top guide, and core plate limit lateral movement of the core fuel bundles.
The shroud, due to its large size, is formed by coupling a plurality of stainless steel cylindrical sections together, typically by welding. Shroud welds, however, increase the susceptibility of the shroud material to a detrimental effect known as inter-granular stress corrosion cracking (IGSCC). Typically, cracking may occur in the heat affected zone of the shroud welds. Currently, volumetric inspections are performed to detect and evaluate the extent of cracking. If the cracking is determined to be significant, repairs may be performed to re-establish the integrity of the weld joint, or the shroud is replaced.
The RPV also includes reactor internal pumps located in the annulus between the shroud and the pressure vessel wall. The internal pumps provide circulation of water in the RPV. Typically differential pressure lines are used to measure the reactor internal pump flow and the flow of water through the reactor core located inside the shroud. The pressure lines are usually constructed using pipe and pipe fittings. The pressure lines enter the RPV through penetrations in the bottom head. The pressure lines extend along the inside of the shroud and are supported by brackets welded to the shroud. The brackets are required to prevent flow induced vibrations in the pressure lines. One reactor internal pump differential pressure line penetrates the shroud above pump impellers and the other terminates below the impellers inside of the shroud. The core differential pressure lines terminate above and below the core plate.
Because the differential lines are welded to the shroud, replacement of shroud sections is difficult and time consuming. The pressure lines must first be removed from the shroud section before replacement of the shroud section. Also the differential pressure lines must be reinstalled, i.e., welded to the new shroud section.
It would be desirable to provide a shroud that includes easily replaceable shroud sections. Particularly, it would be desirable to provide a shroud that includes replaceable shroud sections that can be removed without cutting pressure lines or pressure line supports, and that does not require welding pressure lines and/or pressure line supports to install a shroud section.